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Question. It looks as though you sanded the outside diameter of the cylinder fins on the belt sander. Did you use a jig to make it perfectly round and centered or did you just eyeball it?

The OD was just eyeballed then tuned up a little by hand with a sanding block. The router bearing was riding on the foam to cut the slots. This doesn't work as well as pin guided router jigs because the foam is fragile and the bearing spins and will melt the foam on contact. I have a simple pin guided jig for the next copies. The jig will make perfect copies and be much safer. Although very useful, over arm routers have seriously injured a lot of people. I don't like getting my hands or fingers even remotely close to those exposed bits and good jigs not only secure the part but also allow extra distance for your hands.

If you're interested in using a table router to make foam patterns, search "inverted pin router". There are attachments that adapt ordinary table routers for sale in most of the woodworking tool catalogues. It's also very useful to have a live Z-axis on your router to plunge to set depth hands free and there are now many companies that make motorized "router lifts". I prefer over arms for pin guided routing because you can see the stock being removed but you can do much of the same in an inverted pin router and the are safer because less of the bit is exposed and what is exposed is often covered by the stock you're machining.

You could make that pattern with just a drill press and Forstener bit, shape the OD as I did on a belt or disc sander, then set a hot wire to cut 1/8" thick annular discs out of the foam donut. The cylinder wall could be made by boring a hole and then making a wooden cylinder to bit in the bore with a little circle/wheel jig....which is just a dowel on a board that you can clamp to the sander table and swing it into the belt while you spin the foam cylinder by hand. Then just glue the pieces all together. Capeesh? It may surprise you how nice the pattern is and how quickly it can be done.

I made a router jig so finned cylinder foamies are easy to come by now. I set it up to use that finger joint bit in the foreground so I could cut three fingers at a time. The finger joint bit cuts tapered fins and I scaled the piece down to match the fin proportions. The bore is now 1 ¼”, the cylinder wall a little over an 1/8”, and the annular fins 3/8” deep tapering from 1/8”at the root to 1/16” at the tip. I can make three foamies in 15 minutes from 2”thick foam, boring a hole with a forstener bit, and mounting them on the jig.

Here’s a few of the first batch. I made a Siamese twin just for grins.

Here’s a close up as machined.

I applied a very thin coat of mud. I just slathered it on and brushed with a small paint brush while it dripped off. This thin, I think it may take one or two more coats but at least they should dry ok.

I'll pour this coated one along with the larger bare foam cylinder in the first go. I'll be melting metal this weekend!

Are you going straight to the pour with the drywall mud? I want to see how that compares to being slurried, burned out and then poured....
jason

Going to pour the larger foamie bare and this one coated in mud at the same time. Lots of experience with mud coated foamies now. Works great. I don't presently have any shell but am about to remedy that. I do have foamies and can have as many as I want in short order so at some point that shouldn't be a problem.

So I sprued up a few of the finned cylinder foamies during the week and had a chance to fire up the furnace today. I think I could have easily just gated into the side of the head and cast it at an angle like the rest of the parts but decided to use a horn gate on the head to see how well it worked running in blind under the head.

When I was machining the first larger pattern before I made the jig, I gouged a few of the fins. So I thought it would be a good test part to see if the thin spots filled. So I cast it first. It filled well and was a good part. You can see where the pattern fin was thin and it seem to cast just as the pattern appeared. This one was not coated; just bare foam in coarse sand.

Seeing how well the sample part poured I just decided to casting the remaining foamies together in the same session. Here they are in my lost foam rig before I slipped the pouring cup on them.

…and here they are as cast. They all filled welled. The larger cylinder and head were bare foam in sand and the smaller cylinder I cut on the router jig was coated with dry wall mud.

Here they are with the sprue and gates removed.

I really didn’t make much effort to improve the foamie’s finish on the smaller cylinder but I did take the time to coat it with several thin layers of drywall mud and must say, it makes for a nice part and think it may modestly help with mold stability. Success came quickly with finned cylinders. Wow, I’m very impressed with the lost foam process.